1. Field of the Invention
This invention relates generally to spectroscopy, and more particularly, to a broad-band heterodyne-detected sum frequency generation (HD-SFG) spectroscopy having high sensitivity to enable surface-selective measurements of vibrational spectra at sub-monolayers surface coverage.
2. Description of the Prior Art
Detection of adsorbed molecules at interfaces is of paramount importance in many areas of chemistry, physics, and biology, ranging from surface functionalization chemistry, electrochemistry, and heterogeneous catalysis, to semiconductor passivation, biofouling, and cell membrane biology. In the past decade, vibrational sum frequency generation (SFG) spectroscopy has emerged as one of the main tools for characterization of the molecular structure and dynamics at interfaces. Its advantages are (1) the richness of the molecular-level information (e.g., orientation and conformation) available from the fingerprint mid-IR spectra and (2) the surface selectivity that allows monolayer sensitivity without introducing fluorescent labels or resorting to surface enhancement techniques which typically require a metal surface.
However, improving the detection limits of SFG below a single monolayer coverage has proven to be a challenge. The main reason is that in its conventional homodyne-detected implementation, the intensity of the coherent second-order nonlinear SFG signal scales unfavorably (quadratically) with the surface coverage N of the analyte molecules:ISFG∝|ESFG|2∝|x(2)|2=N2|β2|2   (1)where x2=Nβ(2) is macroscopic nonlinear susceptibility of the surface expressed through the molecular hyperpolarizability β(2) averaged over the orientational distribution at the interface. This implies that detection of 10% of a monolayer entails a factor of 100 decrease in the SFG signal intensity compared to a close-packed monolayer, essentially prohibiting vibrational SFG spectroscopy of samples significantly below monolayer coverage (note, in order to gain a factor of 100 in the signal-to-noise ratio, ×104 longer acquisition would be required at the same laser intensity).
Optical heterodyne detection has been demonstrated to enhance sensitivity of many coherent spectroscopies for bulk-phase studies.
There is a need, therefore, for a heterodyne-detected HD-SFG spectroscopy which overcomes the limitations of the conventional (homodyne-detected) SFG technique.